Probe tip ultrasonic transducers and method of manufacture

ABSTRACT

An improved ultrasonic transducer for a catheter tip has a thin strip of piezoelectric polymer film formed into a spiral ring and adhesively mounted on the support structure near the catheter tip. Electrical connection between the back face of the film and the support structure negative electrode is via capacitive coupling. Connection to the front face of the film is via a wire connected to the positive electrode of the catheter. A further embodiment suitable for a needle transducer is formed by coating the tip with a solution of PVDF co-polymer to form the actual transducer.

BACKGROUND AND PRIOR ART

This invention relates to the field of ultrasonic transducer for usewith probes. Examples are transducers associated with catheter tips foruse with cardiac pacing and electrophysiologic study leads that areimplanted temporarily or permanently within a body, and needle tiptransducers useful for biopsy and catheterization. The invention alsorelates to the probes themselves and methods for their manufacture.

External and implanted cardiac pacemakers are widely used to diagnoseand treat a broad class of cardiac arrhythmias. The electrical pulsesfor heart stimulation are applied to the heart muscle via flexible leadswith active electrodes on their tips. Electrical signals of the heartcan be measured using the same technique with multipolar leads forelectrophysiologic studies.

Cardiac catheterization requires the accurate positioning of a catheterwithin the heart. Positioning is commonly accomplished through the useof an x-ray system. Recently there has been interest in the use oftwo-dimensional ultrasonic echocardiography to localize the cathetertip. Usually an ultrasonic acoustic transducer is mounted at the tip ofthe catheter to allow location of catheter tip employing standardultrasonic sensing equipment. Such equipment operates by employing atransducer to detect ultrasonic radiation emitted from the equipment andreflected from tissue under study. It is also known to emit ultrasonicenergy from other sources and detect it at the equipment transducer. Asthe catheter is inserted into the patient's body and into the scan planeof an external ultrasound imaging probe, the ultrasound energy isdetected by the catheter tip transducer. Based on the elapsed timebetween transmission of a pulse from the imaging probe and the receptionof an echo by the catheter tip transducer, it is possible to calculateand display the exact position of the tip of the catheter on the displayunit of the imaging probe.

A convenient way to ensure that the catheter tip transducer isomnidirectional is to use a tubular shaped transducer mounted on thewall and near the tip of a catheter. A tubular-shaped transducer has anomnidirectional radiation pattern in the plane perpendicular to the axisof the transducer. Piezoelectric polymer film is often used astransducer material because it is thin, flexible and can be made intocomplex shapes. Common types of piezoelectric film are PVDF or PVDFco-polymer. Use of such films has, however, encountered manufacturingdifficulties since the polymer film is difficult to roll into tubularform because of the small dimension of the film. Typically, the edge ofthe film tends to lift up. Further, it is difficult to ensure that theadhesive material is uniformly distributed between the catheter surfaceand the PVDF film.

In various biopsy and catheterization procedures, accurate localizationof the tip of the biopsy needle or a catheter is required. A number ofmethods have been proposed that involve the use of ultrasound imagingfor locating the tip.

Basically this requires mounting one or more transducers on the tip ofthe instrument. By using an ultrasound imaging probe, the patient's bodyis examined to locate the point of interest and guide the instrument tipto that position.

Jan Lesny and Joseph Aindow (UK Patent GB2 157 828 issued on Mar. 4,1987), described a method to mount a transducer at the end of a needletip. In this invention, a small PVDF element is mounted on the centralconductor by means of a small dab of silver loaded epoxy. A thin film ofvarnish is then applied to the edge of the element to ensure electricalisolation of the front and back faces of the PVDF element. A thin filmof gold is then sputtered over the entire outer surface of the needle toprovide electrical conductivity. A protective layer of one or more metallayers is then electroplated over the sputtered layer.

There are several difficulties with this design. Firstly, the PVDFelement mounted on the central conductor is very small (on the order of100 micron), therefore it is very difficult to handle and positionaccurately on the tip of the needle. Secondly, it is very difficult touse a dab of silver loaded epoxy to connect to one electrode of the PVDFfilm (the back electrode) and at the same time ensure that the epoxy isnot smeared or contact is made with the other electrode of the PVDF film(the front electrode). Contact with both the front and back electrodesby the silver loaded epoxy causes a short in the PVDF film and rendersit useless. This is a severe problem because the PVDF film is very thin,in the order of 8-30 micron thick.

Vilkomerson (U.S. Pat. No. 4,407,294 issued Oct. 4, 1987) described amethod to determine the location of a needle tip by mounting a number ofhemispherical transducers on the opposite sides of the tip of anelongated stylet. Again, this technique is extremely difficult becauseof the need to mount the hemispherical transducers onto the tip of thestylet.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel cardiac catheter inwhich the transducer completely encases the catheter core or thesupporting structure of the catheter and a simplified method for itsconstruction.

It is an object of the present invention to provide a convenient methodto manufacture the transducer at the tip of a needle.

It is a further object of the present invention to provide a convenientmethod to manufacture the transducer at the tip of a catheter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a strip of PVDF wound on a forming rod.

FIG. 2 is a view of strip of PVDF wound on a larger diameter rod and cutlongitudinally.

FIG. 3 shows the mounting of the PVDF film coil on the catheter tip.

FIG. 4 shows a catheter attached to an ultrasonically visible tip.

FIG. 5 shows a cross section of an improved needle tip transducer on astylet.

FIG. 6 shows a cross section of an improved catheter tip transducer.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In accordance with the principle of the present invention, a PVDFtransducer is constructed. The process begins by cutting a piezoelectricfilm, preferably PVDF into an approximately 0.7-1 mm width strip. Asshown in FIG. 1, the PVDF film strip 14 is spirally wound onto a formingrod 15. The strip 14 is clamped in place and heat treated at about60°-80° C. to cause the piezoelectric film to retain its spiral form.

As shown in FIG. 2, the preformed spiral film may be removed from theforming rod, and inserted over another, slightly larger in diameter rod16. This rod may have the same diameter as the intended catheter tiponto which the film will be fit. The spiral is then clamped and cutlongitudinally to produce single coil transducers.

The piezoelectric coil is mounted on the intended support 17 which has adiameter slightly larger than the forming rod. The film then fitstightly on the support. A low viscosity adhesive 3 is applied betweenthe gap of the spiral ring to provide adhesion to the support rod. Itshould be understood that single or multicoil spiral film can beproduced in this way and used.

Electrical connection to the inner surface of the spiral ring isaccomplished via capacitive coupling through the adhesive layer 3.Electrical connection to the outer surface of the film is achieved byconnecting a wire to the film using a conductive epoxy or anothermechanical contact technique well known in the art.

The exposed surface of catheter tip adjacent to the mounted transduceris then coated with low viscosity insulator 4 to form an insulating filmsame thickness as the transducer. The catheter tip carrying thetransducer is then connected to the main part of the catheter 5, forexample by crimping 11 over lead 10 to make contact and attach the tip.The area of the crimp is then insulated with a low viscosity insulator.The top face of the transducer is then connected to the other (positive)electrode 6 of catheter via a previously inserted metal coil 8 usingconductive epoxy 12 and 7 to make electrical contact and secure the coilto the tip 13 and the electrode 6 respectively.

This construction is then insulated by molding flexible ultrasonicallytransparent material, such as elastomer or any other, to form a tubularshell 9 in line with the outer surface of the main part of the catheter5.

As a further embodiment of the present invention there is disclosed animprovement in the manufacturing of the needle tip transducer.Basically, as shown in FIG. 5, a coaxial stylet 18 is constructed withthe inner conductor 20 slightly protruded 26 from the outer conductor 22and spaced therefrom by a coaxial insulator 24. The stylet is dipped ina solution of piezoelectric polymer such as PVDF co-polymer of VDF andTrFe in Methylethylketone solution (MTK). The MTK can be evaporated offand the P(VDF-TrFe) crystallized surface 28 formed on the surface of tipof the stylet.

There are various ways to control the thickness of the co-polymer layer28. They are well known in the art. One method is to spin the needle ata high speed so that the centrifugal force results in a uniform layer onthe tip of the needle.

There are certain advantages using this method.

Firstly, there is no need to use conductive adhesive to connect the filmto the stylet. Electrical connection is achieved by capacitive couplingof the film to the inner conductor of the coaxial stylet. Secondly, theprocedure can be performed quickly and does not require a high level ofmanual skill.

The front electrode can also be put on by conventional sputtering orvacuum deposition which are well known in the art. The transducer thencan be poled by a corona discharge or other technique well known in theart (poling is used to make the polymer piezoelectric). A similartechnique can be used to apply a transducer on the tip of a catheter.

The technique is not limited to the PVDF co-polymer but is applicable toall soluble piezoelectric polymers, and is not limited by the size andshape of the coaxial stylet.

Furthermore, the same principle can be applied to produce a transducerat the tip of a catheter 30 or an electrophysiological lead, as shown inFIG. 6. The tip of the lead 34 may be dipped into a solution of PVDFco-polymer and the polymer allowed to crystallize forming a PVDFcopolymer layer 32. Electrical connection to the inner side of thetransducer is via a central conductor 36 and capacitive coupling betweenthe catheter tip 34 and the piezoelectric layer 32. The outer electrode38 can be put on the surface of the piezoelectric film by conventionaltechniques well known in the art. Connection to the outer electrode isvia connector 40. The connector is buried in a protective layer 42,which encloses catheter wall 44.

What is claimed is:
 1. A biological probe enclosing first and secondconductors and having a tip, the probe comprising an ultrasonictransducer formed from a piezoelectric material,said piezoelectricmaterial having the form of a spiral ring about said probe tip, saidspiral ring having inner and outer surfaces, said inner surface of saidpiezoelectric spiral ring being capacitively coupled to said firstconductor, said outer surface of said piezoelectric spiral ring beingelectrically coupled to said second conductor.
 2. The biological probeof claim 1 wherein said piezoelectric spiral ring is connected to saidprobe tip by a low viscosity adhesive and said inner surface of saidpiezoelectric spiral ring is capacitively coupled through said lowviscosity adhesive to said first conductor.
 3. The biological probe ofclaim 2 further comprising an insulator coating said tip and forming aninsulating film having the same approximate thickness as said spiralring.
 4. The biological prove of claim 3 having a flexible lead portionwith an outer generally cylindrical surface and further comprising anultrasonically substantially transparent insulator forming a shellhaving an outer surface approximately in line with the outer surface ofthe probe.
 5. A biological probe having a flexible lead portion with anouter generally cylindrical surface enclosing inner and outer coaxialconductors and a tip, the probe comprising an omnidirectional ultrasonictransducer formed from a piezoelectric material,said piezoelectricmaterial having the form of a spiral ring of width approximately 0.7 to1.0 mm wide about said probe tip and connected thereto by a lowviscosity adhesive, said spiral ring having inner and outer surfaces,said inner surface of said piezoelectric spiral ring being capacitivelycoupled through said low viscosity adhesive to said inner coaxialconductor, said outer surface of said piezoelectric spiral ring beingelectrically coupled to said outer coaxial conductor, an insulatorcoating said tip and forming an insulating film having the sameapproximate thickness as said spiral ring, an ultrasonicallysubstantially transparent insulator forming a tubular shell having anouter surface approximately in line with said outer surface of theprobe.